* For ozone, the maximum of the 8 hourly and hourly mean is used to calculate the index value.

Pollution concentrations recorded by the analysers have been classified according to Defra's
Air Pollution Index system. This classes levels into bands from 'low' to 'very high'. Each band is subdivided into three to produce
an Air Pollution Index, from 1 to 10, 1 being 'low', 10 being 'very high'. The concentrations are classified according to
the table above. Note that different pollutants have different concentrations and averaging periods, related to the estimated health effects of each.